Method for producing carbohydrate partial esters

ABSTRACT

A process for making carbohydrate partial esters having a degree of esterification of from 1 to 6 involving the steps of: (a) providing a catalytically active mixture containing: (i) an alkali metal carbonate; and (ii) a fatty acid lower alkyl ester; (b) providing an emulsifier mixture containing: (iii) a glycose component having from 5 to 12 carbon atoms; and (iv) a carbohydrate partial ester; and (c) combining the catalytically active mixture with the emulsifier mixture, with vigorous stirring, to form an emulsion/dispersion containing particles having a mean diameter of from 10 to 60 μm.

BACKGROUND OF THE INVENTION

This invention relates to an improved process for the production ofcarbohydrate partial esters by transesterification of glycoses withfatty acid esters in the presence of emulsifiers and basic catalysts andto the use of the substances obtainable by the process for a number ofapplications.

Carbonhydrate esters, which are often also referred in short as “sugaresters”, are esters of mono- or oligosaccharides and—in the broadersense—of sugar alcohols with organic or inorganic acids. Carbohydrateesters have pronounced surface-active properties so that, today, theyare regarded as an independent class of compounds (so-called sugarsurfactants). By virtue of their favorable dermatological andtoxicological compatibility, carbohydrate esters are mainly used asemulsifiers for the production of foods and cosmetics. Sucrosepolyesters containing 6 to 8 fatty acid residues could be used as a fatsubstitute which is not utilized by the organism in the diet ofoverweight people and, in addition, are said to bind LDL cholesterol inthe stomach. They are normally produced by subjecting glycoses totransesterification with fatty acid methyl esters in the presence ofalkaline catalysts and optionally emulsifiers. One such process isdescribed, for example, in German patent application DE-A14131505(Henkel). Soaps are generally used as emulsifiers for the production ofthe carbohydrate esters, as disclosed for example in GB-A 2,256,869.

Unfortunately, known processes are attended by the disadvantage thatlong reaction times are required to achieve in particular relativelyhigh degrees of esterification, which makes the products expensive onaccount of the considerable reactor possession times. In addition, theproducts are often discolored on account of their prolonged exposure toheat. Finally, solvents often have to be used which not only addsfurther to the cost of the products on account of the subsequent removalof the solvents used, but also is often undesirable if the products areto be used, for example, in the food sector.

Accordingly, the problem addressed by the present invention was toprovide an improved process for the production of carbohydrate partialesters which would be free from the disadvantages mentioned above.

DESCRIPTION OF THE INVENTION

The present invention relates to a process for the production ofcarbohydrate partial esters with a degree of esterification of 1 to 6 byalkali-catalyzed transesterification in the presence of emulsifiers,characterized in that

(a) to form a catalytically active system, alkali metal carbonates aretreated with fatty acid lower alkyl esters corresponding to formula (I):

R¹CO—OR²  (I)

in which R¹CO is a linear or branched, saturated or unsaturated alkylgroup containing 6 to 22 carbon atoms and R² is a linear or branchedalkyl group containing 1 to 5 carbon atoms, and

(b) for transesterification, the resulting mixture is treated whilestirring vigorously with a mixture of (b1) glycoses containing 5 to 12carbon atoms and (b2) carbohydrate partial esters as emulsifiers, sothat an emulsion/dispersion in which the particles have a mean diameterof 10 to 60 μm is obtained.

In the first step of the process according to the invention, a coatingof fatty acid lower alkyl ester is formed on the alkali metal carbonate,so that the acyl group is activated. In the second step, the activatedcatalyst is contacted with a mixture of a glycose and a carbohydratepartial ester, the carbohydrate partial ester acting as an emulsifier.In the first step of the transesterification, acyl groups aretransferred to the emulsifier which, in the further course of thereaction, itself functions as an acylating agent and transfers acylgroups to the glycose which is thus converted into a carbohydratepartial ester. The reaction takes place in the absence of solvents whichis a considerable advantage not only from the economic point of view,but also with a view to the use of the end product in foods orcosmetics. Another unexpected advantage of the process is that, byvirtue of the effective nature of the transesterification, the samedegrees of esterification are obtained in very much shorter reactiontimes by comparison with the prior art.

Catalysts

Suitable catalysts are alkali metal carbonates, preferably sodium and/orpotassium carbonate, which may be used in quantities of 5 to 50% byweight, preferably 10 to 20% by weight and more preferably 10 to 15% byweight, based on the fatty acid lower alkyl esters.

Fatty Acid Lower Alkyl Esters

Typical examples of suitable acylating agents are the esters of caproicacid, caprylic acid, 2-ethylhexanoic acid, capric acid, lauric acid,isotridecanoic acid, myristic acid, palmitic acid, palmitoleic acid,stearic acid, isostearic acid, oleic acid, elaidic acid, petroselicacid, linoleic acid, linolenic acid, elaeostearic acid, arachic acid,gadoleic acid, behenic acid and erucic acid and technical mixturesthereof with methanol, ethanol, propanol, isopropyl alcohol, n-butanol,i-butanol, tert.butanol, n-pentanol and isopentanol. Methyl and/or ethylesters of C₁₂₋₁₈ fatty acids are preferably used, the molar ratio ofester to glycose being from 1:3 to 3:1 and preferably from 1:2 to 2:1,depending on the required degree of esterification.

Production of the Catalyst System

To produce the catalyst system, i.e. to activate the fatty acid alkylester as acylating agent, the ester and the alkali metal carbonate aremixed with intensive stirring. It has proved to be of advantage in thisregard to carry out the activation at temperatures of 40 to 120° C. andpreferably 80 to 100° C. The quantity of alkali metal carbonate used ispreferably in the range from 5 to 50% by weight, based on the alkylester. The ester undergoes chemisorption onto the surface of thecarbonate.

Glycoses

Glycoses in the context of the invention include thepolyhydroxyaldehydes (aldoses) and polyhydroxyketones (ketoses) alsoreferred to as carbohydrates and relatively high molecular weightcompounds which can be converted into such substances by hydrolysis.According to the invention, both monomeric polyhydroxyaldehydes orpolyhydroxyketones (monosaccharides) and their dimers to decamers(disaccharides, trisaccharides, oligosaccharides) may be used asglycoses. Suitable monosaccharides (also known as “simple sugars”) are,for example, bioses, trioses, tetraoses, pentoses, hexoses, heptoses,etc. Typical examples of aldopentoses are D-ribose, D-xylose andL-arabinose. The most important aldohexoses include D-glucose, D-mannoseand D-galactose while the ketohexoses include D-fructose and sorbose.The 6-deoxysugars, L-fucose and L-rhamnose, are also widely used hexosesand are also suitable as starting materials. The simplestoligosaccharides suitable as starting materials are the disaccharides.Sucrose (cane sugar, beet sugar), lactose (milk sugar) and/or maltose(malt sugar) are preferably used. According to the invention it ispreferred to mono- and/or disaccharides, sucrose or glucose beingparticularly preferred.

Emulsifiers

According to the invention, it has proved to be of particular advantageto use carbohydrate partial esters which are identical with the targetproducts as emulsifiers. It is particularly preferred to use sucrosepartial esters with a degree of etherification of 1 to 3. Other suitableco-emulsifiers are, for example, nonionic surfactants from at least oneof the following groups:

(1) products of the addition of 2 to 30 mol of ethylene oxide and/or 0to 5 mol of propylene oxide onto linear fatty alcohols containing 8 to22 carbon atoms, onto fatty acids containing 12 to 22 carbon atoms andonto alkylphenols containing 8 to 15 carbon atoms in the alkyl group;

(2) C_(12/18) fatty acid monoesters and diesters of products of theaddition of 1 to 30 mol of ethylene oxide onto glycerol;

(3) glycerol monoesters and diesters and sorbitan monoesters anddiesters of saturated and unsaturated fatty acids containing 6 to 22carbon atoms and ethylene oxide adducts thereof;

(4) alkyl mono- and oligoglycosides containing 8 to 22 carbon atoms inthe alkyl group and ethoxylated analogs thereof;

(5) adducts of 15 to 60 mol of ethylene oxide with castor oil and/orhydrogenated castor oil;

(6) polyol esters and, in particular, polyglycerol esters such as, forexample, polyglycerol polyricinoleate or polyglycerolpoly-12-hydroxystearate. Mixtures of compounds from several of theseclasses are also suitable;

(7) products of the addition of 2 to 15 mol of ethylene oxide ontocastor oil and/or hydrogenated castor oil;

(8) partial esters based on linear, branched, unsaturated or saturatedC_(6/22) fatty acids, ricinoleic acid and 12-hydroxystearic acid andglycerol, polyglycerol, pentaerythritol, dipentaerythritol, sugaralcohols (for example sorbitol), alkyl glucosides (for example methylglucoside, butyl glucoside, lauryl glucoside) and polyglucosides (forexample cellulose);

(9) trialkyl phosphates and mono-, di- and/or tri-PEG-alkyl phosphates;

(10) wool wax alcohols;

(11) polysiloxane/polyalkyl polyether copolymers and correspondingderivatives;

(12) mixed esters of pentaerythritol, fatty acids, citric acid and fattyalcohol according to DE-PS 1165574 and/or mixed esters of fatty acidscontaining 6 to 22 carbon atoms, methyl glucose and polyols, preferablyglycerol or polyglycerol, and

(13) polyalkylene glycols.

The addition products of ethylene oxide and/or propylene oxide withfatty alcohols, fatty acids, glycerol monoesters and diesters andsorbitan monoesters and diesters of fatty acids or with castor oil areknown commercially available products. They are homolog mixtures ofwhich the average degree of alkoxylation corresponds to the ratiobetween the quantities of ethylene oxide and/or propylene oxide andsubstrate with which the addition reaction is carried out. C_(1/18)fatty acid monoesters and diesters of adducts of ethylene oxide withglycerol are known as refatting agents for cosmetic formulations fromDE-PS 20 24 051. C_(8/18) alkyl mono- and oligoglycosides are producedin particular by reacting glucose or oligosaccharides with primaryalcohols containing 8 to 18 carbon atoms. So far as the glucoside unitis concerned, both monoglycosides in which a cyclic sugar unit isattached to the fatty alcohol by a glycoside bond and oligomericglycosides with a degree of oligomerization of preferably up to about 8are suitable. The degree of oligomerization is a statistical mean valueon which the homolog distribution typical of such technical products isbased. According to the invention, however, it has proved to be ofparticular advantage to use carbohydrate partial esters identical withthe target products as emulsifiers. The carbohydrate ester emulsifiersand the glycoses are used in a molar ratio of 1:6 to 6:1 and preferably1:5 to 5:1. The percentage content of the co-emulsifiers may be in therange from 5 to 50% by weight and is preferably in the range from 10 to25% by weight, based on the emulsifiers.

Transesterification

The transesterification is carried out by preparing anemulsion/dispersion containing the catalyst system, the glycose and thecarbohydrate ester and optionally other emulsifiers with vigorousstirring. The emulsion/dispersion has a mean particle size of 10 to 60μm and preferably 20 to 40 μm. The transesterification takes place inthis emulsion/dispersion at temperatures of preferably 100 to 160° C.and more preferably 120 to 140° C. and under a reduced pressure of 1 to10 mbar and preferably 2 to 8 mbar, the reaction times being in therange from 1 to 4 h. Any alcohol released may readily be distilled offin vacuo. Carbohydrate partial esters with a degree of esterification ofpreferably 1 to 6 and more preferably 1 to 3 are obtained in this way.

Commercial Applications

The carbohydrate esters obtainable by the process according to theinvention have excellent surface-active properties and may be used, forexample, as emulsifiers for the production of foods (bread,confectionery, ice cream, etc.) and cosmetic preparations in which theymay be present in quantities of 0.1 to 10% by weight and preferably 1 to5% by weight. The carbohydrate esters are particularly suitable asemulsifiers for the production of polyacrylic or polymethacrylic acidcompounds which may be used as superadsorbers, for example for diapers.Since the emulsifiers remain in the product, not only their excellentperformance properties, but also their particular dermatologicalcompatibility is of particular relevance. Accordingly, the presentinvention also relates to the use of the carbohydrate esters obtainableby the process according to the invention as emulsifiers for theproduction of foods, cosmetic products and superadsorbers.

EXAMPLES Example 1

47.5 g (0.2 mol) of hydrogenated palm oil fatty acid methyl ester wereintroduced into a 500 ml three-necked flask equipped with a high-speedstirrer, reflux condenser and dropping funnel and heated to 65° C. 5 g(0.035 mol) of potassium carbonate, corresponding to 12% by weight,based on the methyl ester, were added at a stirring speed of 1000 r.p.m.10 g (0.02 mol) of an ester of sucrose with palm oil fatty acid (degreeof esterification ca. 2) were then added in portions to the resultingdispersion through the dropping funnel. After stirring for 15 minutes,the stirring speed was increased to 2000 r.p.m. and 37.5 g(0.1 mol) ofsucrose were added (molar ratio of methyl ester to sucrose=2). Theresulting emulsion/dispersion was then stirred for another 4 hours at300 r.p.m., at a temperature of 120° C. and under a reduced pressure of7 mbar. A sucrose ester containing 15% by weight of monoester, 25% byweight of diester and 60% by weight of higher homologs, based on thequantity of carbohydrate ester, was obtained. The content of unreactedester in the reaction mixture was below 1% by weight while the contentof free sucrose was 6% by weight.

Example 2

Example 1 was repeated using 56.8 g (0.2 mol) of tallow fatty acidmethyl ester, 10 g of potassium carbonate (corresponding to 18% byweight, based on methyl ester), 15 g (0.03 mol) of sucrose ester and51.3 g (0.15 mol) of sucrose (molar ratio of methyl ester tosucrose=1.3). A sucrose ester containing 11% by weight of monoester, 29%by weight of diester and 60% by weight of higher homologs, based on thequantity of carbohydrate ester, was obtained. The content of unreactedester in the reaction mixture was below 1% by weight while the contentof free sucrose was 7% by weight.

What is claimed is:
 1. A process for making carbohydrate partial estershaving a degree of esterification of from 1 to 6 comprising: (a)providing a catalytically active mixture containing: (i) an alkali metalcarbonate; and (ii) a fatty acid lower alkyl ester corresponding toformula (I): R¹CO—OR²  (I) wherein R¹CO is a linear or branched,saturated or unsaturated acyl group having from 6 to 22 carbon atoms,and R² is a linear or branched alkyl group containing from 1 to 5 carbonatoms; (b) providing an emulsifier mixture containing: (iii)a glycosecomponent having from 5 to 12 carbon atoms; and (iv) a carbohydratepartial ester; and (c) combining the catalytically active mixture withthe emulsifier mixture, with vigorous stirring, to form anemulsion/dispersion containing particles having a mean diameter of from10 to 60 μm.
 2. The process of claim 1 wherein the alkali metalcarbonate is selected from the group consisting of sodium carbonate,potassium carbonate, and mixtures thereof.
 3. The process of claim 1wherein in formula (I), R² is a methyl group, an ethyl group, or both.4. The process of claim 1 wherein the catalytically active mixture isprepared at a temperature of from 40 to 120° C., with intensivestirring.
 5. The process of claim 1 wherein the glycose component isselected from the group consisting of monosaccharides, disaccharides,and mixtures thereof.
 6. The process of claim 1 wherein the carbohydratepartial ester is a sucrose partial ester having a degree ofesterefication of from 1 to
 3. 7. The process of claim 1 wherein thecatalytically active mixture and the emulsifier mixture are combined ata temperature of from 100 to 160° C., and a reduced pressure of from 1to 10 mbar.
 8. The process of claim 1 wherein the alkali metal carbonateis used in an amount of from 5 to 50% by weight, based on the weight ofthe fatty acid lower alkyl ester.
 9. The process of claim 1 wherein theprocess is conducted in the absence of any solvents.
 10. The process ofclaim 1 wherein the glycose component and carbohydrate partial ester areemployed in a molar ratio of from 1:6 to 6:1.